In automatic transmissions, such as dual clutch transmissions, one or more hydraulically-operated actuators is/are often used for shifting the gears. In particular, such a hydraulically-operated actuator may be a hydraulic cylinder having a shift piston that can reciprocally move back and forth and/or up and down in a hollow chamber of a work cylinder. In such automatic transmissions, a shift rod is typically disposed within a shift cylinder and is affixed to the shift piston.
Shifting devices are also known that include a stepped piston, which is sealingly guided in a hollow chamber and/or work cylinder by a shift rod. The stepped piston can be affixed to the shift rod, e.g., with retaining rings. Sealing means are typically provided on the shift piston for sealing the two work chambers of the work cylinder relative to each other, such as e.g., seals vulcanized onto forged metallic piston elements. A similar shifting device includes stationary seals that seal against the piston rods, which are movable relative to the seals, and separate the two oil chambers. However, in such known embodiments, the seals can not reliably separate the two oils from each other.
Since different oil environments, i.e. different operating oils, can be utilized in different work chambers in modern automatic transmissions, in particular in dual clutch transmissions, it is necessary to reliably seal the work chamber of the work cylinder relative to the outer environment(s) in order to prevent the different oils from mixing together. For example, two oils may be utilized that are incompatible with each other, such as a hydraulic oil (e.g., automatic transmission fluid) for the hydraulic operation and a gear oil for the clutch package. In this case, the two oils may be present on opposite sides of the hydraulic piston.
Oil carryover or leakage past the seals and piston guides should be avoided due to the oil incompatibility or should at least be limited to an absolute minimum. Maintaining the necessary oil separation becomes even more challenging when the piston has an annular shape (annular piston).